1. Field of the Invention
The present invention relates to a surface light source device of side light type, more specifically, relates to a surface light source device of side light type in which uniformity of luminance is improved. The present invention is applied to back lighting of a liquid crystal display device, for example.
2. Related Art
It is know that a surface light source device of side light type provides a back lighting arrangement of thin type for lighting a liquid crystal display panel. In general, the surface light source device of side light type has a light guide plate which is a plate member composed of a light guide material, and a primary light source including a rod-type light source for supplying an illumination light to the light guide plate sideways.
The illumination light emitted from the primary light source is guided into the light guide plate through an end surface of the light guide plate (incidence surface). The guided illumination light is refracted to be emitted from one major surface of the light guide plate to a liquid crystal display panel. Since the primary light source is arranged beside the light guide plate, the whole device can be thinned easily.
As, the light guide plate employed in the surface light source device of side light type, a plate having substantially uniform thickness, and a plate whose thickness tends to become smaller as the distance from the primary light source increases, are known. In general, the latter plate emits an illumination light more efficiently than the former plate.
FIG. 4 is an assembly perspective view showing a general structure of the surface light source device of side light type using the latter light guide plate. FIG. 5 is a cross section taken along line A--A in FIG. 4. Referring to both the drawings, a surface light source device of side light type 1 has a light guide plate 2, and a primary light source 3 is arranged besides the light guide plate 2. The surface light source device 1 further has a reflection sheet 4, a diffusible sheet 5, and prism sheets 6 and 7 which serve as light control members, and they as well as the light guide plate 2 are laminatedly arranged as shown in FIG. 5.
The primary light source 3 has a cold cathode tube (fluorescent lamp) 8 and a reflector 9 which surrounds the cold cathode tube 8. The reflector 9 has an opening, and supplies an illumination light to the end surface 2A of the light guide plate 2 through the opening. The reflector 9 is composed of a regular reflection or irregular reflection sheet material, etc. The light guide plate 2 with wedge-shaped cross section is formed by the injection molding method of acrylic (PMMA) resin, for example. The light guide plate 2 receives the illumination light of the primary light source 3 from the incidence surface 2A which is one end surface.
The light guided into the light guide plate 2 is simultaneously reflected and propagated repeatedly between a major surface along the reflection sheet 4 (hereinafter, referred to as "slanted surface") 2B and another major surface along the diffusible sheet 5 (hereinafter, referred to as "emitting surface") 2C.
In every reflecting time, a component having an incidence angle not more than a critical angle is emitted from the slanted surface 2B and emitting surface 2C. Therefore, the main propagating direction of the illumination light emitted from the emitting surface 2C is slanted to a direction of the point of the wedge shape. This is called directional emitting characteristic.
Further, a light diffusible surface 2D is formed on the slanted surface 2B. A diffusible characteristic which is increased from the incidence surface 2A to the point of the wedge shape is given to the light diffusible surface 2D. The diffusible characteristic is given by application of diffusible ink which includes pigment such as magnesium carbonate and titanic oxide. A degree of the diffusible characteristic can be adjusted according to an amount of application of the diffusible ink.
Instead of by the application of the diffusible ink, by forming the slanted surface 2B on a satin finished surface (rough surface), the diffusible characteristic may be given. Also in this case, the diffusible characteristic is given so as to be increased gradually towards a direction from the incidence surface 2A to the point of the wedge shape. For this reason, a region of the rectangular, for example, the satin finished surface is formed at constant or random pitch so as to be distributed with high density towards the direction from the incidence surface 2A to the point of the wedge shape.
This light guide plate 2 corrects lowering of emission quantity in the vicinity of the point of the wedge shape, and makes a quantity distribution of the emitted light uniform. Here, a point to which special attention should be paid is that the diffusible characteristic is not enough strong to lose emission directivity of the light guide plate. Namely, even when the diffusible characteristic is given to the slanted surface 2B, the main propagating direction of the illumination light emitted from the emitting surface 2C is slanted to the direction of the point of the wedge shape.
The reflection sheet 4 is formed by a sheet-type regular reflection member composed of metallic foil, etc. or a sheet-type irregular reflection member composed of white PET film, etc. The illumination light leaking from the slanted surface 2B is reflected by the reflection sheet 4 to enter the light guide plate 2 again, and as a result, the use efficiency of the illumination light is improved.
The prism sheets 6 and 7 are arranged in order to correct the emission directivity of the light guide plate 2. The diffusible sheet 5 is arranged so as to prevent the light diffusible surface 2D of the slanted surface 2B from visually recognizing from an upper part of the emitting surface 2c, and to make brightness and shadow, etc. of each portion of the light guide plate 2 which is irradiated by the illumination light unnoticeable.
The diffusible sheet 5 diffuses the illumination light emitted from the light guide plate 2. The prism sheets 6 and 7 are formed by transparent sheet materials such as polycarbonates. The prism surface is formed on a surface (outer surface) which is opposite with the light guide plate 2. This prism surface is composed of a plurality of protrusions having rectangular cross section which are extended substantially parallel with one direction. In this example, the prism sheet 6 on the inner side is aligned that its protrusions are extended parallel with the incidence surface 2A, and the prism sheet 7 is aligned so that its protrusions are extended to a direction which intersects perpendicularly to the incidence surface 2A.
The prism sheets 6 and 7 correct the main emitting direction of the emitted light to a frontal direction of the emitting surface 2C on the slanted surface of each protrusion. A so-called dual-side prism sheet in which the prism surfaces are formed on both the surfaces may be used.
In general, the surface light source device of side light type adopting the wedge-shaped light guide plate and prism sheets can emit an emitting light to the frontal direction more efficiently than the surface light source device of side light type adopting the light guide plate having substantially uniform thickness.
Such a surface light source device of side light type causes following one problem in connection with an electrode of the rod-type light source (fluorescent lamp 8). In general, the fluorescent lamp 8 as the rod-type light source has electrodes 8A and 8B at its both ends. Regions of the inner wall of the tube to which a fluorescent material is not applied exist in the vicinities of the electrodes 8A and 8B. For this reason, regions which do not emit illumination lights L efficiently are formed in the vicinities of both the ends of the fluorescent lamp 8. A portion other than these regions (center or its circumference) provides an efficient light emitting region which emits an illumination light efficiently.
When a length H of the incidence surface 2A of the light guide plate 2 is larger than a length of the fluorescent lamp 8, as represented by B in FIG. 4, an emission quantity is lowered at the side end of the incidence surface, and thus nonuniformity of luminance occurs.
According to a conventional solving method, as shown in FIG. 7, the incidence surface 2A having a longer length H than the fluorescent lamp 8 requires a light scattering surface 2D in the vicinities of the electrodes 8A and 8B where light scattering power is strong locally. For this reason, an ink adhesion area which is large per unit area is provided to the vicinities of the electrodes 8A and 8B. As a result, in the light guide plate 2, the emission from the emitting surface is promoted in the vicinities of the end surfaces, and lowering of the emitting light intensity is supplemented.
FIGS. 6 and 7 illustrate distribution of the light diffusible surface (ink adhesion portion) 2D on the slanted surface of the conventional light guide plate 2. The line C--C represents a cross line in the vicinity of the incidence surface 2A. Attention should be paid to that in both the light guide plates 2, the length of the incidence surface 2A is slightly smaller than the length of the fluorescent lamp 8.
In the case of FIG. 6, the ink adhesion area per unit area is increased simply according to a distance from the incidence surface 2A. A curved line LA in FIG. 8 represents the luminance of the emitting surface along the line C--C. As is clear from this, in this case, the emission quantity on the side of the incidence surface 2A represented by line C--C is lowered greatly in the vicinities of both the ends of the incidence surface.
On the contrary, as shown in FIG. 7, when the light scattering power is increased locally in the vicinities of the electrodes 8A and 8B, as shown by a curved line LB in FIG. 8, the luminance in the circumference of the end portions is increased.
However, even if the light guide plate 2 show n in FIG. 7 is used, the problem is not sufficiently solved, and the emission quantity (luminance) is more insufficient in the vicinities of the electrodes 8A and 8B than the central portion.